Improving groundwater–surface water exchange representation in WALRUS
Evaluation of direction- and temperature-dependent resistance parameterization for a managed dutch lowland catchment
S.H.G. Barendregt (TU Delft - Civil Engineering & Geosciences)
M. Hrachowitz – Mentor (TU Delft - Civil Engineering & Geosciences)
R.R.P. van Nooijen – Graduation committee member (TU Delft - Civil Engineering & Geosciences)
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Abstract
Climate change is intensifying hydrological extremes in Dutch lowland catchments, where shallow groundwater tables, active water management, and strong groundwater–surface water (GW–SW) coupling pose particular modelling challenges. This thesis investigates whether targeted structural modifications to the Wageningen Lowland Runoff Simulator (WALRUS) can improve its performance for the managed lowland catchment of the Marswetering in the Netherlands.
Two adaptations are proposed and evaluated against the baseline model. The first is a temperature- and direction-dependent groundwater resistance parameterization (cG), which is intended to represent streambed clogging asymmetry as well as seasonal variability driven by changes in water viscosity. The second is an adapted quickflow partitioning formulation (fQS), which is designed to prevent the complete suppression of land-surface quickflow during extended dry periods. Five model variants are calibrated using a multi-objective framework combining six hydraulic signatures including discharge, flow duration curve, autocorrelation, seasonal runoff coefficient, and normalized groundwater dynamics, and evaluated across a calibration period and two independent testing periods.
All model variants reproduce peak discharge with reasonable skill, but consistently underperform on low flows and summer groundwater dynamics. A structurally recurring opposite-sign error in summer groundwater behaviour, present across all variants and both testing periods, points to systematic underestimation of actual evapotranspiration or outward seepage or overestimation of the equilibrium storage deficit. The adapted fQS formulation produces a physically consistent but negligible improvement, largely absorbed by compensating changes in the quickflow reservoir constant. The direction-dependent cG formulation successfully reproduces asymmetric GW–SW exchange resistance, but introduces elevated summer discharge that worsens seasonal water balance closure. The temperature-dependent component of cG shows no identifiable calibrated influence. Additionally, the spatially uniform representation of managed surface water levels is identified as a source of uncertainty, given the heterogeneous water management across the catchment's sub-catchments.
The results indicate that improving summer unsaturated zone dynamics represents a higher priority for WALRUS development than further GW–SW exchange complexity. The multi-objective calibration framework developed here is directly transferable to comparable lowland catchment applications.